4.1.2.13 Minimum Implementation; RFC-959 Section 5.1

4.1.2.13 Minimum Implementation; RFC-959 Section 5.1

The following commands and options MUST be supported by
every server-FTP and user-FTP, except in cases where the
underlying file system or operating system does not allow or
support a particular command.

Vendors are encouraged to implement a larger subset of
the protocol. For example, there are important
robustness features in the protocol (e.g., Restart,
ABOR, block mode) that would be an aid to some Internet
users but are not widely implemented.

A host that does not have record structures in its file
system may still accept files with STRU R, recording
the byte stream literally.